No species in nature kills its own kind more often or more creatively than humans do, yet we cannot seem to devise a reliably swift, painless method of capital punishment.

Oklahoma’s bungled execution of Clayton Lockett is the latest death-chamber debacle. After receiving a supposedly lethal injection, the convicted murderer began writhing, mumbling and tried to rise off the gurney.

Officials later said that the procedure caused a “vein failure.” The executioner who was administering the fatal dose might have been unaware, since he was separated from Lockett by a wall.

That’s not standard procedure in hospitals and medical offices. Usually the person giving the injection is standing next to the patient, not hiding from him.

When Lockett squirmed back toward consciousness, the execution was stopped. Prison officers said he died of a heart attack 27 minutes later. By that time the blinds to the chamber window had been shut to prevent the witnesses from seeing Lockett’s continued suffering.

He’d received the death penalty for a hideous crime, shooting a woman and burying her alive. Most of those who now say he didn’t suffer enough have never attended an execution. I have.

The faces of those who supervised Lockett’s final moments were ashen when they emerged. Torture is bad policy in capital cases because the Constitution outlaws “cruel and unusual” punishment. The underlying moral precept is that the state shouldn’t act as savagely as the person who committed the murder.

Every flubbed execution gives more weight and momentum to the legal challenges mounted by opponents of capital punishment. Not only are death sentences handed out inconsistently, and not only have many innocent persons been convicted (and later been exonerated after decades on Death Row), but the very act of execution clearly hasn’t be engineered to make it humane and instantaneous.

One of many nationwide nicknamed “Old Sparky,” Florida’s electric chair was retired in 2000 after several cinematic screw-ups. Real sparks and smoke came from the face mask of Jesse Tafero while he was being executed for the murder of a Highway Patrol trooper. Years later, inmate Pedro Medina’s head actually caught fire while he was strapped in the chair.

In both cases, prison officials said Old Sparky had functioned flawlessly, and blamed the unexpected combustion on sponges that were placed beneath the inmates’ death caps. There was much debate about whether the men died before, during or after the scorching.

The chair was eventually rebuilt to support the bulk of Allen Lee Davis, a monster who’d murdered a pregnant woman and her children in Jacksonville. In 1999 he became the last person to die by state electrocution in Florida.

Blood dripped from Davis’ nose during the procedure. An autopsy also revealed burns to his head, leg and groin, the gruesome death photos provoking such worldwide outrage that the state mothballed the electric chair and switched to lethal injection.

To a conflicted public it seemed like a better solution — just give the bad guy a shot and put him to sleep forever. Veterinarians do it all the time to ailing dogs and cats.

But lethally injecting humans hasn’t gone as smoothly as advocates of capital punishment had predicted. It’s difficult to find anesthesiologists and other medical professionals who will participate in the killings, and often lay persons are employed to administer the lethal doses.

In one of two incidents during 2006, a misplaced catheter prolonged for half an hour the consciousness — and agony — of Florida Death Row inmate Angel Diaz. Last January, an Ohio convict gasped audibly for more than 10 minutes before the injections began to work.

And yet another Oklahoma inmate moaned during his recent execution: “I feel my whole body burning.” You might believe he deserved that pain, and more, but the law of the land stands firmly against such emotions.

Like many states, Oklahoma had used a sequence of substances beginning with a heavy sedative. The drugs were made mostly in Europe, where capital punishment is banned in almost every country. Some of the drug manufacturers have taken steps to prevent their products from being used in American executions.

Consequently, Oklahoma has been experimenting with secret new intravenous combinations. Attorneys for defendants facing the death penalty have been trying to stop executions there until the mixture is revealed, and tested for its effects on humans.

Whatever toxic cocktail was shot into Clayton Lockett’s arms, it failed in a demonstrably cruel and unusual way to do the job.

The twisted irony is that officials halted his execution on the chance that he might fully awaken. Then his ruptured vein would heal and they could try to kill him again on another day, without screwing it up so badly.